The present invention relates to signal processing, and more particularly to the subtraction of wide band signals, such as high definition television video signals, for observing timing errors between various components of the signals.
For many applications the subtraction of signals is desirable. For example in the television arts it is desirable to check for timing errors between the signal components of a video signal over an entire horizontal video line, and the subtraction of component signals is one method for providing a visual display on a waveform monitor of such errors. However the subtracting of signals becomes more difficult at higher frequencies due to the need to match phase as well as amplitude over a wide band of frequencies. Any circuit component mismatches introduce various parasitic impedances that are frequency dependent, especially parasitic capacitances introduced by active electronic devices such as transistors.
Subtraction of signals is commonly achieved by inputting the two signals to be subtracted to a differential amplifier, one signal being input to the inverting input and the other to the non-inverting input. To achieve the desired subtraction accuracies at higher frequencies the transistors that make up the differential amplifier have to be well matched and the common mode rejection has to be very good. Typical specifications for common mode rejection are listed at different frequencies, such as 60 Hz, 15 kHz and 4.5 MHz. At 60 Hz and 15 kHz the common mode rejection error typically is 0.5%, but at 4.5 MHz the common mode rejection error becomes 2%. For high definition television video signals with a wide band signal up to 30 MHz, the common mode rejection error becomes unacceptable.
Therefore what is desired is a method of subtracting wide band signals without introducing large frequency dependent errors into the result.